When a vehicle having such a seat installed therein is subjected to a front impact, the seat belt applies a large force to the torso of the seat user, to such an extent that the seat belt is in danger of injuring the user.
Naturally, the same phenomenon exists when the seat belt is not "built in" the seat, but is secured to the bodywork of the vehicle.
However, in that case, it is known that the phenomenon can be remedied by fixing the anchor points of the seat belt to sheet metal that is tearable, as described, for example, in an article published in the journal "Le Journal de l'Automobile" (No. 483/484 of Apr. 28, 1995, page 8). Thus, when the vehicle is subjected to a front impact, the sheet metal tears progressively under traction force from the seat belt, enabling the anchor point of the belt to move over a certain distance, which may be as much as 30 cm, for example. In this way, the movement of the user's torso in a forward direction is braked progressively, thereby putting a limit on the forces exerted by the seat belt on the user's torso. This avoids injuring the user.
When the seat belt is "built in" the seat, the above technical solution is inapplicable. The space available inside the seat is too restricted to enable one of the anchor points of the seat belt, as incorporated in the seat, to be subjected to linear displacement of significant magnitude, e.g. about 30 cm.